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Old 02-17-2017, 07:04 AM   #101
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I found a very accurate fuel consumption Nordig 37-39 Tug boat test made. I was surprised how low consuptions you can go, if necessary, NT 7 nautical miles 1 gal of about 6 knots. I like Boat test site because they have such a specialist factual.


link which you interested NT precise consumption figures.


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Old 02-17-2017, 07:45 AM   #102
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"Hi Smitty
During my research I came across a engine research facilty, very comprehensive, thats were I came across the most easily understood research info spreasheets etc. You could plug in air temp, speed, altitude, air pressure a huge load of varibles. from this I was able to increase my HP at a given RPM to obtain better combustion , by the engines ingesting colder and denser air. Previously when in the open sea I would notice that the tachs would dip slightly as seas passed underneath the boat etc. from this we were aware we were not getting constant BHP (it was dipping and rising as the props came under load) due mostly by imgesting hot air at 130F. The calcs proved that the BHP would increase to near original Lehman test figures if we could get the temps down to under 100F. Previously the engines were ingesting hot thin air and making up for it by increasing the fuel and we were using higher RPM to make the speed"



Hello Martin J,


It is possible that you can make more hp at a particular rpm by adding colder intake air. It is possible that you are making better mpg by any number of methods.
But you cannot make better speed at a set rpm unless you are making changes to a propeller, transmission or the boats hull and/or running gear is cleaner.

You posted this relationship about rpm and speed again here....
"we were using higher RPM to make the speed"
RPM and HP produced in a diesel are not related closely at all. RPM and fuel use is mostly dependent upon the load on the engine and not the rpm.
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Old 02-17-2017, 07:53 AM   #103
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"I found a very accurate fuel consumption Nordig 37-39 Tug boat test made. I was surprised how low consuptions you can go, if necessary, NT 7 nautical miles 1 gal of about 6 knots. I like Boat test site because they have such a specialist factual."

I do not know anything about that particular model or the resultant numbers of fuel use vs speeds. But the relative % changes between 9 knots and lower speeds down to 4 knots is pretty consistent with just about any diesel boat I have seen. In other words it is typical that very large increases in distances covered per gallon occur with fairly small decreased in speed when you are under 9-10 knots. Often you can make very large changes in fuel use by going just a wee bot slower when at these speeds. It often makes boat comparisons hard as the data is not typically available to compare these details.
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Old 02-17-2017, 08:15 AM   #104
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Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


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Old 02-17-2017, 09:19 AM   #105
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Originally Posted by Tony Athens View Post
Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


Tony
Absolutely. In big head seas my smooth water average speed of say 7.9 knots at 1700 RPM will drop below 6.5 knots at same RPM. Climbing a large ocean swell shows the same, going uphill as they say. Then throw in 25 knots of breeze on the nose
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Old 02-17-2017, 11:51 AM   #106
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Originally Posted by Tony Athens View Post
Not to discount all of the tests I have seen like this, but none of them take into account "real ocean conditions".........................Add a light average 2-3 ft head seas and 10-15 Kts of wind and the numbers would drop at a least 25% over a 24 hour run.. Put some nasty weather (up or down swell) is there and things really change on a 100 mile run...

But it is nice to know that when it's flat glass conditions, you can get 3 - 6 MPG running at hull type speed (or a tad less) in a 20 ton vessel..


Tony
They all state clearly what the conditions are and you are correct they aren't done in nasty conditions. Now, the average person here doesn't boat in nasty conditions.
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Old 02-17-2017, 12:23 PM   #107
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Now, the average person here doesn't boat in nasty conditions.
Correct, but ----. One person's average may well be another's extreme.
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Old 02-17-2017, 12:30 PM   #108
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Correct, but ----. One person's average may well be another's extreme.
Very true. And Tony's right about the dangers of just taking numbers and not looking at situations. We have usage numbers over the life of our boats at various speeds. But none of those necessarily reflect the conditions we will face tomorrow.

I ran across someone in a panic because his fuel consumption was so much worse than he'd seen on performance charts. He'd just come south, the entire trip in the Gulf Stream, he'd had a strong wind against him the entire way, and he's been in 6-8' seas the entire trip. I realized how much trouble he was in though when I found out he didn't know the Gulf Stream flowed South to North. He had no idea the current against him.
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Old 02-17-2017, 05:05 PM   #109
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I have only one long journey, this nordic tug. time to her home in the northern German into last fall. about 600 nautical miles and an average speed of 15 knots and I can say that the consumption of the same, as the test boat, but in 2600 The motor spins a little smaller than the test. wave was all the way from 5-10 feet swollen. upcoming boating season when the sea ice cover has melted to give me the whole truth. It's great that Tony seaboard commented here in the forum, I just got a new raw water pump him.
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